1. Field of the Invention
This invention relates to color matching and more particularly to a process and related apparatus for matching the color displayed on a plurality of color display devices.
2. Description of Related Art
The primary use of this invention is in the field of computer assisted color publishing systems particularly in the area of color matching. In such systems typically a color image is scanned using a scanning device which measures light intensity reflected or transmitted from the image to generate electronic signals representing three achromatic images of the original, each of the three achromatic images resulting from detecting the light intensity from the original color image through a colored filter. The filters used are almost always Red, Green and Blue. The electronic signal from the scanner is converted to a digital signal in which light intensity levels are represented as numbers. Information identifying each set of numbers representing the image information obtained through each filter is also preserved. Thus, through these steps, the original colored image is converted to a plurality of image values, and for each picture element in the picture, there are three such values, a Red, a Green and a Blue.
The image represented by the image values may be displayed in a CRT type monitor, or may be printed using a printing device able to accept electronic input. Often the image will be displayed at different locations and times using more than one display device, such as a plurality of CRT displays for observation and study by more than one operator. Similarly, hard copies may be desired in two or more different locations using two or more different printers. Even though the input values to the multiple CRT displays are the same, the displayed colors are vastly different, as anyone is well aware of who has ever observed the multiple television displays in a typical television sales store front. However, when one needs to make decisions about acceptability of color for a display, one needs to know with a great degree of confidence that the color, that one is observing and discussing with an operator who is observing the same image on a different CRT, is the same as the color observed by the operator. The same is true, if the displayed image is one created on a printer and compared with the same image created on a different printer.
Colorimetry, which is the study of color based on both spectral distribution of the energy reflected or transmitted from a test sample and the response of the human eye, as well as the spectral distribution of the illuminating source, provides a method to describe and measure color and enables one to determine when colors match. Through the use of CIE defined Tristimulus Values (which are the amounts of three primary lights which when added produce a visual, or colorimetric match with an original color), one may determine with reasonable certainty that if two colors have the same three CIE Tristimulus values that is, if the Red Tristimulus value of one color is the same as the Red Tristimulus value of the other, and so on for the Green and Blue Tristimulus values, then the appearance of the two colors will be the same to the average observer. CIE stands for the International Commission on Illumination.
It appears, therefore, that the problem of matching color outputs of different displays is readily resolved by providing displays having the same CIE tristimulus values for the same image value inputs.
While the solution in principle is simple, creating a conversion device which will make two or more different displays produce the same tristimulus output values for the same input image values is very difficult. Each of the display devices operates in its own, device dependent, color space where image values at its input are transformed into display image values. The difficulty lies primarily in matching the two transformations occurring within the two display devices for the image values appearing at the input of each, so that both display the same displayed colorimetric values for the same input image values.
The prior art solutions to color matching fall into two fundamentally distinct approaches. The first is based on the decomposition of a color vector to a set of primaries, and known as the primary decomposition technique. Typical of this approach is the technique described in U.S. patent application 5,196,927 filed Mar. 16, 1990, assigned to E. I. du Pont de Nemours and Company. According to this technique, the input image is decomposed into the linear combination of eight primaries (yellow, magenta, cyan, black, red, green, blue, and three-color black). Coefficients for a 4.times.8matrix (the values of CYMK for each of the eight primaries) are adjusted such that the color of the eight primaries in the input from both systems to be matched are matched, and the new input values are found by matrix multiplication of an 8.times.1 matrix (coefficient of decomposition) with the 4.times.8 matrix. This technique suffers because of the non linearity in the additive properties of color dyes and because the "primaries" used are not mathematically independent.
The more common solution, adopted by the printing industry is the grid sampling technique. This involves using a color transformation formula such that the error between the targeted color and the processed color is minimized. The differences between the various methods in existence are found in the specific transformation formulas. U.S. Pat. No. 4,500,919 issued to Schreiber is a good example of the use of a transformation formula. The difficulty with this approach again lies in the non-linearity of the color addition process and the complexity of the color surfaces in a set of equations.